As it is well known, network re-configurations, link or node failures, protection switching, and add/drop multiplexing may cause abrupt changes of the power levels of signals propagating in optical telecommunications networks. This may cause degradation in signal-to-noise ratios (SNR) and increase in bit error rates (BER). To compensate for the unexpected power variations, a variable optical attenuator (VOA) is usually inserted (coupled with power monitoring and microcontroller apparatus) in the path of the incoming signal for each wavelength and the attenuator's setting is adjusted to a pre-determined fixed value which may or may not be sufficient to reduce and eliminate the unexpected signal power fluctuations. Generally, the VOA control loop mechanism is not fast enough to provide the required attenuation to prevent damage to downstream optical components in the optical network, which can occur on the sub-millisecond timescale. For example, a newly added channel may pass through a downstream optical amplifier that services a plurality of optical channels. The newly added channel may instantaneously have too much optical power, until the VOA can adjust it to a pre-determined fixed value. In the meantime, the amplifier gain will be reduced until its own internal control loop can compensate for the excess input power by increasing its signal power, in a constant gain amplifier. The plurality of optical channels momentarily suffers a drop in optical power, which may lead to excessive BER on these channels. The attenuation of the VOA is not always adequate in this case to eliminate the power spikes that could damage the downstream optical components and cannot be changed fast enough to avoid the damage.
FIG. 1 shows a flow diagram 100 for a simple prior art VOA attenuation setting algorithm, wherein upon start up (block 105) routine 107 initializes the VOA attenuation (block 110) using a target power determined during commissioning of installed system (block 170) to maintain a constant mean output power in a closed loop manner. Routine 107 uses the actual power (block 120) measured by the power monitoring (block 180) to adjust the VOA attenuation setting when power level changes.
If measured power is greater than target power (block 130) routine 107 increases VOA attenuation (block 135). Routine 107 provides the new value (block 160) to the microcontroller to adjust VOA attenuation setting. If measured power is less than target power (block 140) routine 107 decreases VOA attenuation (block 145). Routine 107 provides the new value (block 160) to the microcontroller to set up the VOA attenuation.
As the input power varies, the VOA attenuation compensates to produce a stable output power. If the input power drops, the VOA attenuation decreases towards zero. If the input power drops to nil, the control algorithm maintains the VOA attenuation at zero.
FIG. 2 shows a flow diagram for another prior art VOA attenuation setting algorithm, wherein upon start up (block 205) routine 207 adjusts the VOA attenuation caused by power changes. Target power (block 270) and low power threshold (block 215) are determined during commissioning of installed system in the network. Routine 207 uses these values to initialize the VOA control loop (block 210). Routine 207 uses actual power (block 220) measured by the power monitor (block 280) to adjust VOA attenuation when power changes are detected. If the measured power is less than a pre-determined threshold value (block 230) routine 207 continues (block 235) with no adjustment to VOA attenuation (block 260) and the microcontroller maintains current VOA attenuation setting. If the measured power is greater than the target power (block 240) routine 207 increases the VOA attenuation (block 245) and continues (block 260) where the microcontroller increases the VOA attenuation setting. If the measured power is less than the target power (block 250) routine 207 decreases VOA attenuation (block 255) and continues (block 260) where microcontroller decreases the VOA attenuation setting. If the power level is greater than a pre-determined threshold value, it stops controlling the VOA and maintains the power level at the current value, at the time the large deviation was detected. The VOA control loop moves into an open loop mode to maintain constant attenuation until power is restored and stabilized.
FIG. 3 shows a flow diagram of a U.S. Pat. No. 6,207,949 entitled, “Method and apparatus for stabilizing attenuators in optical networks” to Jackel, J., issued on Mar. 27, 2001, for a pre-determined fixed VOA attenuation setting algorithm. This operates the VOA at a pre-determined fixed value less than the minimum attenuation, wherein upon start up (block 305) routine 307 sets the VOA attenuation to a pre-determined fixed value less than the minimum attenuation whenever a loss of incoming signal power is detected. The target power (block 370) and low power threshold (block 315) are determined during commissioning of installed system in the network. Routine 307 initializes the VOA control loop (block 310) with the target power and low power threshold values. Routine 307 uses actual power (block 320) measured by the power monitoring (block 380) to adjust the VOA attenuation setting. If measured power is less than pre-determined threshold (block 330) routine 307 sets the VOA attenuation to a pre-determined fixed value of less than the minimum attenuation (block 335) and continues (block 360) where the microcontroller sets the VOA attenuation level to a pre-determined fixed value of less than the minimum attenuation. If the measured power is greater than the target power (block 340) routine 307 increases the VOA attenuation (block 345) by a pre-determined fixed value of less than the minimum attenuation and continues (block 360) where the microcontroller sets the VOA to the new value. If measured power is less than target power (block 350) routine 307 decreases VOA attenuation (block 355) by a pre-determined fixed value of less than the minimum attenuation and continues (block 360) where microcontroller sets the VOA attenuation to the new value.
The pre-determined fixed attenuation value of less than the minimum attenuation reduces some power spikes in the optical network. However, this pre-determined fixed attenuation value of less than the VOA minimum attenuation may not be sufficient to eliminate the power spikes occurred in the optical network that cause optical channel signal-to-noise degradation and increase in bit error rate, and the attenuation may not be changed fast enough to avoid these problems.
Similarly, a U.S. Pat. No. 6,304,347 entitled, “Optical power management in an optical network” to Beine, T., et al, issued on Oct. 16, 2001, teaches a system for managing signal power levels in an optical network where power parameters information exchanges and re-configuring of the nodes may cause abrupt changes of the power levels of the signal propagating in the optical network that may cause degradation in the SNR (signal-to-noise ratios) and BER (bit error rates). The prior art teaches of a VOA control loop operates in an open loop and closed loop modes. When the VOA control loop is open, the VOA attenuation is set to a pre-calibrated fixed value. FIG. 4 shows a typical VOA attenuation response 410, in terms of insertion loss attenuation in dB 405 and bias 415, the pre-calibrated fixed value 420 is equal to the default insertion loss for a default input 430 of the VOA. When the VOA control loop is closed, attempts are made to hold the output power for the VOA constant for changes at the input. A control algorithm, similar to that of U.S. Pat. No. 6,207,949 and is shown in FIG. 3, is implemented to determine the switching between closed loop and open loop VOA operations. Upon start up (block 305) routine 307, representing the prior art VOA control loop algorithm used by the microcontroller, when the VOA control loop is in open mode, sets the VOA attenuation to a pre-calibrated fixed value whenever a loss of incoming signal power is detected. Routine 307 initializes the VOA control loop (block 310) using target power (block 370) and low power threshold (315) determined during commissioning of the installed node in the optical network. Routine 307 uses actual power (block 320) measured by the power monitoring (block 380) to determine the required attenuation setting for VOA. If measured power is less than pre-determined threshold (block 330) routine 307 sets the VOA attenuation to a pre-calibrated fixed value and continues (block 360) where the microcontroller sets VOA attenuation to the pre-calibrated fixed value. If measured power is greater than target power (block 340) routine 307 increases the VOA attenuation (block 345) by a delta determined from input and output power measurements and continues (block 360) where the microcontroller increases the VOA setting by the new delta. If measured power is less than target power (block 350) routine 307 decreases VOA attenuation (block 355) by a delta determined using input and output power measurements and continues (block 360) where the microcontroller decreases the VOA setting by the new delta. The deltas are fixed values based on input and output power measurements.
Unfortunately, the teaching of U.S. Pat. No. 6,304,347, where the VOA attenuation is set at a pre-calibrated fixed value and fixed deltas, may or may not be sufficient to eliminate power spikes which occur in the optical network and which cause degradation in SNR and excessive BER, and it cannot be changed sufficiently fast in order to avoid these problems.
Accordingly, there is a need for the development of improved methods and devices for power control in optical systems and networks, which would avoid and reduce the shortcomings and limitations of the prior art.